SO 30885883 - Why does this Reflex code result in Dynamics firing indefinitely with the same value?

so30885883.hs

-- http://stackoverflow.com/questions/30885883/why-does-this-reflex-code-result-in-dynamics-firing-indefinitely-with-the-same-v
{-# LANGUAGE RecursiveDo, ScopedTypeVariables #-}

import Reflex
import Reflex.Dom

import qualified Data.Map as Map

dynButton
  :: MonadWidget t m
  => Dynamic t String
  -> m (Event t ())
dynButton s = do
  (e, _) <- el' "button" $ dynText s
  return $ _el_clicked e

-- widget that takes dynamic list of strings
-- and displays a button for each, returning
-- an event of chosen button's index
listChoiceWidget
  :: forall t m. MonadWidget t m
  => Dynamic t [String]
  -> m (Event t Int)
listChoiceWidget choices = el "div" $ do
  asMap <- mapDyn (Map.fromList . zip [(0::Int)..]) choices
  evs :: Dynamic t (Map.Map Int (Event t ())) <- listWithKey asMap (\_ s -> dynButton s)
  dynEv :: Dynamic t (Event t Int) <- mapDyn (leftmost . Map.elems . Map.mapWithKey (\k e -> fmap (const k) e)) evs
  return $ switch (current dynEv)

options :: MonadWidget t m => Dynamic t Int -> m (Dynamic t [String])
options foo = do
  mapDyn (\x -> ["a", "b", show x]) foo

main :: IO ()
main = mainWidget $ el "div" $ do
  rec n <- listChoiceWidget o
      o <- options foo
      foo <- holdDyn 0 n
  display (traceDyn "foo" foo)